1. Field of the Invention
The present invention relates to a liquid crystal display element displaying images utilizing a cholesteric liquid crystal layer having memory characteristics and a method of driving the element.
2. Description of the Related Art
Recently, various enterprises and universities are actively engaged in the development of electronic paper. Various applied portable apparatus have been proposed as promising markets of applications of electronic paper, including electronic books which are the most promising of all, sub-displays of mobile terminal apparatus, and display sections of IC cards. One of advantageous display methods for electronic paper is the use of a display element utilizing a liquid crystal composition (a cholesteric liquid crystal) in which a cholesteric phase is formed. A cholesteric liquid crystal has advantageous characteristics such as semi-permanent display retention characteristics (memory characteristics), vivid color display characteristics, high contrast characteristics, and high resolution characteristics.
A cholesteric liquid crystal has bi-stability (memory characteristics), and the liquid crystal can be put in any of a planar state, a focal conic state, or an intermediate state that is a mixture of the planar state and the focal conic state by adjusting the intensity of an electric field applied to the same. Once the liquid crystal enters the planar state, or the focal conic state, the state is thereafter maintained with stability even if there is no electric field.
The planar state can be obtained by applying a predetermined high voltage to a liquid crystal to apply a strong electric field to the same and thereafter nullifying the electric field abruptly. For example, the focal conic state can be obtained by applying a predetermined voltage lower than the high voltage to the liquid crystal to apply an electric field to the same and thereafter nullifying the electric field abruptly. The intermediate state that is a mixture of the planar and focal conic states can be obtained by, for example, applying a voltage lower than the voltage to obtain the focal conic state to the liquid crystal to apply an electric field to the same and thereafter nullifying the electric field abruptly.
Principles of display operations of a liquid crystal display element utilizing such a cholesteric liquid crystal will now be described with reference to FIGS. 14A and 14B using a B display portion 46b for displaying blue as an example. FIG. 14A shows alignment of liquid crystal molecules 33 of the cholesteric liquid crystal observed when a B liquid crystal layer 43b of the B display portion 46b is in the planar state. FIG. 14B shows alignment of the liquid crystal molecules 33 of the cholesteric liquid crystal observed when the B liquid crystal layer 43b of the B display portion 46b is in the focal conic state.
As shown in FIG. 14A, in the planar state, the liquid crystal molecules 33 are sequentially rotated in the direction of the thickness of substrates to form a helical structure, and the helical axis of the helical structure is substantially perpendicular to the substrate surfaces. In the planar state, light rays having predetermined wavelengths in accordance with the helical pitch of the liquid crystal molecules are selectively reflected by the liquid crystal layer. A wavelength λ at which maximum reflection takes place is given by λ=n·p where n represents the average refractive index of the liquid crystal layer and p represents the helical pitch.
Therefore, in order to allow blue light to be selectively reflected by the B liquid crystal layer 43b of the B display portion 46b in the planar state, the average refractive index n and the helical pitch p are determined, for example, such that an equation “λ=480 nm” holds true. The average refractive index n can be adjusted by selecting the liquid crystal material and the chiral material appropriately, and the helical pitch p can be adjusted by adjusting the chiral material content.
As shown in FIG. 14B, in the focal conic state, the liquid crystal molecules 33 are sequentially rotated in an in-plane direction of the substrates to form a helical structure, and the helical axis of the helical structure is substantially parallel to the substrate surfaces. In the focal conic state, the B liquid crystal layer 43b loses the selectivity of wavelengths to be reflected, and most of incident light rays are transmitted. Since the transmitted light rays are efficiently absorbed by a light-absorbing layer disposed, for example, on a bottom surface of a bottom substrate 49b of the B display portion 46b, a dark state (black) can be displayed.
As thus described, reflection and transmission of light by the cholesteric liquid crystal can be controlled by helically twisted states of alignment of the liquid crystal molecules 33. Cholesteric liquid crystals selectively reflecting green and red light in the planar state are enclosed in a G liquid crystal layer 43g for displaying green and an R liquid crystal layer 43r for displaying red, respectively, in the same manner as done for the B liquid crystal layer 43b, whereby a display section capable of full-color display is fabricated.
FIG. 15 shows examples of reflectance spectra observed at the B, G, and R liquid crystal layers when they are in the planar state. Wavelengths of reflected light rays are shown (in nm) along the horizontal axis, and reflectances are shown along the vertical axis (in percents in comparison to the reflectance of a white plate). The curve connecting the triangular symbols in the figure represents the reflectance spectrum observed at the B liquid crystal layer 43b. Similarly, the curve connecting the square symbols in the figure represents the reflectance spectrum observed at the G liquid crystal layer 43g, and the curve connecting the rhombic symbols in the figure represents the reflectance spectrum observed at the R liquid crystal layer 43r. 
As shown in FIG. 15, since the center wavelengths of the reflectance spectra of the B, G, and R liquid crystal layers in the planar state have magnitudes ascending in the order in which the liquid crystal layers are listed. The helical pitches of the cholesteric liquid crystals of the B, G, and R liquid crystal layers have magnitudes ascending in the order in which the liquid crystal layers are listed. Therefore, the chiral material contents of the cholesteric liquid crystals of the B, G, and R liquid crystal layers must have magnitudes descending in the order in which the liquid crystal layers are listed.
In general, liquid crystal molecules must be twisted stronger to make the helical pitch thereof smaller, as the wavelength to be reflected is shorter. Then, the cholesteric liquid crystal of interest consequently has a high chiral material content. It is also true in general that a higher chiral material content tends to necessitate a higher driving voltage. A cholesteric liquid crystal has a greater reflection bandwidth Δλ, as the refractive index anisotropy Δn of the liquid crystal is greater.
A liquid crystal display element utilizing a cholesteric liquid crystal has the property of memorizing a state of display. The element can semi-permanently retain a state of display of an image even when no electric power is supplied and can display the image in the memorized state of display. Therefore, such an element is suitable for applications in which the same memorized image is to be displayed for a long time. However, such a liquid crystal display element has the problem of so-called image sticking that is faint persistence of a previous image occurring when the previous image is rewritten into a new image after being displayed for a long time.
It is assumed that image sticking is attributable to factors such as affinity between moisture, ionic impurities or a liquid crystal and substrate interfaces. Approaches to the mitigation of such image sticking have been proposed, including a refreshing method which involves a sequence having the steps of applying a voltage to a cholesteric liquid crystal such that the alignment of the cholesteric liquid crystal substantially becomes parallel to the voltage applying direction at regular time interval and thereafter applying a voltage associated with display data (see Patent Document 1).
There is another proposed method for preventing image sticking, including the steps of converting image data of an image using an NOT element when a predetermined period passes after the image is displayed and displaying another image based on the data obtained by the conversion (see Patent Document 2).
There is still another proposed method for preventing image sticking, including the steps of acquiring information on the ambient temperature of a liquid crystal display element and displaying a black image for preventing image sticking when a temperature change per unit time representing a temperature rise of a predetermined value or more is detected (see Patent Document 3).    Patent Document 1: JP-A-2002-14325    Patent Document 2: JP-A-2004-4200    Patent Document 3: JP-A-2004-219715
However, the degree of image sticking can vary in a complicated manner depending on the accumulation of image sticking of past images, the period for which the same image has been continuously displayed, and the influence of ambient temperature. It is therefore difficult to prevent image sticking completely using the above-described methods.